ProjectConfig().to_json() produces object instead of array

[cyril-cpm]

Change behavior of ProjectConfig().to_json() to produce real object instead of array (with key as index 0 and value as index 1).
Example:

platformio project config --json-output:

{
  "env:test": {
    "build_flags": [
      "-D FLG_A",
      "-D FLG_B=1"
    ],
    "monitor_speed": 115200
  },
  "env:test2": {
    "build_flags": [
      "-D FLG_A",
      "-D FLG_B=1"
    ],
    "monitor_speed": 115200
  }
}

instead of:

[
  [
    "env:test",
    [
      [
        "build_flags",
        [
          "-D FLG_A",
          "-D FLG_B=1"
        ]
      ],
      [
        "monitor_speed",
        115200
      ]
    ]
  ],
  [
    "env:test2",
    [
      [
        "build_flags",
        [
          "-D FLG_A",
          "-D FLG_B=1"
        ]
      ],
      [
        "monitor_speed",
        115200
      ]
    ]
  ]
]